Doped alumina catalysts

ABSTRACT

Described is a catalyst for the removal of pollutants from the exhaust gases of automotive internal combustion engines wherein the catalyst is alumina doped with cations of two groups of metals M and M′ being present in a quantity in the range 10 to 25% by weight of the catalyst and the weight ratio of M:M′ lies in the range 0.5-2.0.

This invention relates to doped alumina catalysts; that is to saycatalysts principally comprising alumina (Al₂O₃) and certain additives.It is particularly concerned with such catalysts for use in the removalof pollutants from the exhaust gases of automotive internal combustionengines.

Catalysts used for treatment of automotive exhaust gases to removecarbon monoxide, hydrocarbons and oxides of nitrogen (NO_(x)) are oftentermed three-way catalysts (TWCs). The efficiency with which theyachieve the removal is affected by such factors as the prevailingtemperature (T) and the air:fuel ratio (λ). They need to operate over arange of temperatures in order to commence operation quickly after acold start, and also to operate effectively at sustained hightemperatures. These high temperatures may result from the catalyst'sproximity to the engine and/or to a strongly exothermic reaction takingplace on the catalyst. Similarly the catalysts need to operate over areasonably wide range of air:fuel ratios.

Alumina is attractive as a TWC component because of its low cost, goodinteraction with precursors of other TWC components, its high surfacearea and its stability to temperatures in excess of 1100° K. It may beapplied as boehmite (AlOOH), which then converts to thethermodynamically stable α-phase of Al₂O₃.

Doping of the alumina with certain metal cations of variable or fixedoxidation state has been widely proposed with a view to providingimproved thermal stability, hardness and reactivity over alumina alone.Metals employed as such dopants include cerium (Ce³⁺/Ce⁴⁺) and barium(Ba²⁺). One or more platinum group metals (referred to herein as PGMs)when supported on Al₂O₃ promote conversion of the exhaust streampollutants; for example oxidation of CO and hydrocarbons mostly underoxygen-rich conditions or reduction-decomposition of NO_(x) mostly underoxygen-lean conditions.

Ceria (CeO₂) is a well-established alumina dopant, typically used in aquantity of up to 20% by weight of the catalyst. At lower proportions(e.g. <1%) and elevated temperatures (e.g. >1200° K) CeAlO₃ can beformed, but at higher ceria contents the Al₂O₃ and CeO₂ tend tosegregate at the Al₂O₃ surface. Ceria can take up and release oxygenreversibly and so is said to have an oxygen storage capacity (OSC) thatcan assist CO and hydrocarbon oxidation under oxygen-lean conditions.

Other materials suggested as oxidation exhaust catalysts includelanthanum cobaltite (LaCoO₃) and barium cerate (BaCeO₃), the latterbeing a proton-conducting perovskite (with each Ba²⁺ surrounded by eightCeO₆ octahedra).

Improvements in three-way catalysts have been achieved in part bycombinations of different dopants. Thus the oxygen storage capacity(OSC) of ceria is enhanced by interaction with any PGMs present. Ceriahas also been found to modify ZrO₂ and its CO and propene oxidationactivity, with the result that CeO₂—Al₂O₃ and CeO₂—ZrO₂ have beenextensively used in automotive exhaust catalysts. Tb₄O₇ improves the OSCof CeO₂, even though the surface area (45 m²/g) can be modest.

Incorporation of ions such as La³⁺ increases the stability of thealumina at high temperatures. BaO has been added to alumina, for exampleby conventional, micro-emulsion and sol-gel methods, leading ultimatelyto hexa-aluminate (BHA; BaAl₁₂O₁₉). Alumina has also been simultaneouslydoped with BaO and CeO₂ [Angrove et al, Appl. Catal. 194-5A,27,(2000)]with analysis of the phases developed.

PGMs commonly used in TWCs have been palladium, platinum and rhodium,typically in concentrations of 0.3 to 1.2 g/dm³ (kg/m³). A three waycatalyst could thus be Pt—Rh/CeO₂—Al₂O₃. Wang et al [Solid State Ionics111,333,(1998)] and Dunn et al [Solid State Ionics 128,141, (2000)]report the insertion of metal cations into tetrahedral and/or octahedralsites in the O²⁻ array in XAl₂O₄, XAl₁₂O₁₉ and X-β-Al₂O₃.

U.S. Pat. Nos. 5,939,354 and 5,977,017 of S J Golden relate to certainperovskite-type catalysts with three-way activity for the removal ofpollutants from exhaust gases of internal combustion engines and fromindustrial waste gases. The catalysts are represented by the generalformula A_(a-x)B_(x)MO_(b), in which A is a mixture of elementsoriginally in the form of a defined mixed lanthanide; B is a divalent ormonovalent cation; M is at least one element selected from the groupconsisting of elements having an atomic number of 22 to 30, 40 to 51,and 73 to 80; a is 1 or 2; b is 6 or 4 when a is respectively 1 or 2;and 0≦x<7.

The catalytic activity of perovskites has also been addressed byJovanovic et al (Three-Way Activity and Sulphur Tolerance of SinglePhase Perovskites; CAPOC II; 1991, page 391 et seq.) andMathieu-Deremince et al (Structure and Catalytic Activity of MixedOxides of Perovskite Structure; CAPOC III; 1995, page 393 et seq.).

Traditionally the catalysts for automotive exhaust treatment have beenapplied to a substrate material, for example by wash-coating. Suitablesubstrate materials include certain ceramics, for example cordierite(2MgO.Al₂O₃.5SiO₂), and certain metals, for example stainless steel,Fecralloy™ and titanium.

Concern may at some time arise over the implications of PGM emissionsfrom automotive catalysts and over the poisoning effect on the platinumgroup metals of materials such as lead, sulphur and phosphorus derivedfrom the fuel. In part as a result of these concerns attempts have beenmade to produce non-PGM catalysts for use in emission control fromautomobile exhausts albeit that these non-PGM catalysts often havemodest surface area. The aim has been to provide non-PGM catalysts thatwould not be poisoned by metals in the exhaust gas stream, for exampleby metals such as lead, manganese, sodium or potassium which derive fromgasoline, or by metals such as zinc derived from lubricating oil, or byother poisons such as sulphur or phosphorus. While it has been foundthat the activity of non-PGM catalysts increases with pollutant contacttime [L. S.Yao [React.Kin.Catal.Lett. 56,283,(1995)] no such catalysthas hitherto been developed that is sufficiently efficient.

The present invention accordingly has the object of producing non-PGMautomotive exhaust catalysts with efficiencies close to those having aPGM content.

According to the invention there is provided a catalyst for the removalof pollutants from the exhaust gases of automotive internal combustionengines which comprises alumina doped with cations of other metals,characterised in that the dopants comprise cations of two groups ofmetals M and M′ wherein M and M′ are:

-   -   (a) monovalent M (e.g. K⁺) and pentavalent M′ (e.g. Ta⁵⁺) or,    -   (b) divalent M (e.g. Ba²⁺) and quadravalent M′ (e.g. Ce⁴⁺) or,    -   (c) divalent M (e.g. Ba²⁺) and a combination of divalent and        pentavalent M′ (e.g. Co²⁺ and Ta⁵⁺) or,    -   (d) divalent M (e.g. Ba²⁺) and a combination of trivalent and        pentavalent M′ (e.g. Ce³⁺ and Nb⁵⁺) or,    -   (e) divalent M (e.g. Ba²⁺) and a combination of divalent and        hexavalent M′ (e.g. Ba²⁺ and Re⁶⁺) or,    -   (f) trivalent M (e.g. Ce³⁺) and trivalent M′ (e.g. Fe³⁺) or,    -   (g) trivalent M (e.g. La³⁺) and a combination of divalent and        quadravalent M′ (e.g. Co²⁺ and Ir⁴⁺)        and wherein M and M′ are both present in a quantity in the range        10 to 25% by weight of the catalyst and the weight ratio of M:M′        lies in the range 0.5-2.0. M and M′ may be selected from those        illustrated by F. S. Galasso in ‘Structure, Properties and        Preparation of Perovskite-type Compounds’ (Pergamon Press, 1969)        and give alumina-dispersed MM′O_(z) (where z is variable around        a value of 3).

A doped alumina catalyst according to the invention is described hereinas a “pairwise-doped alumina”. This catalyst can be prepared as acomposite or homogenous phase by sol-gel routes as further describedbelow to give a “sol-gel pairwise-doped alumina” (SPA). The primarybeneficial effect of the M dopants lies in promoting CO oxidation butthese also assist in promoting hydrocarbon oxidation. In contrast, theprimary beneficial effect of the M′ dopants lies in promotinghydrocarbon oxidation but these also assist in promoting CO oxidation.The combined use of both dopants provides a synergistic improvement overeither type of dopant used alone, to the extent of meeting the objectiveof achieving pollutant removal efficiencies comparable with catalystshaving a PGM content.

The oxygen content of the catalysts according to the invention variesaccording to the prevailing air:fuel ratio, temperature and time. Theirlead content varies with the nature and levels of lead in the fuel, thestream composition, time and the prevailing temperature (especiallysince they will operate at higher temperatures than PGM-catalysts). Thishigher temperature lowers the level of lead uptake.

Another specific advantage of catalysts according to the invention isthat they have good thermal stability, thereby permitting their use asclose-coupled catalysts.

A further advantage of the catalysts is that they have strong resistanceto poisoning by lead, manganese, sodium or potassium from gasoline, orby zinc from lubricating oil, or by other poisons such as sulphur orphosphorus. In terms of the effect upon the catalyst of the inventionPb—PbO_(x) is not a simple poison and indeed the PbO_(x) is at times apromoter rather than a poison. These Pb—PbO_(x) and PbO_(x) materialsenter and leave the catalyst without long-term damage to its activity.

Thus unlike PGM catalysts, the materials of the invention are notdestroyed by lead introduced from the fuel, nor do they emit PGMs duringuse. The catalysts are therefore of special benefit in markets where thelead content of gasoline is high enough to have a detrimental effect ona catalyst over its useful life. Even a lead content of 5 mg/dm³ hasbeen found to have a noticeably harmful effect on conventional Pt:RhTWCs. It is therefore common practice for automotive manufacturers toincrease the PGM loading to ensure that the required durability criteriaare met.

Emission standards for vehicles in developed countries are becomingincreasingly demanding, with the effect that even a small degree ofpoisoning or inhibition of a catalyst by trace elements present in thegasoline or lubricating oils is quite undesirable. There is also a trendfor the relevant government agencies to increase the minimum life spanover which a catalyst must remain effective, and some legislation nowrequires a catalytic converter fitted to a new vehicle to keep withinofficial emission limits for 150,000 miles. In order to meet theseexpectations of lower emission levels coupled with increased durability,there is a perceived need either completely to eliminate known poisonsfrom gasolines and lubricating oils, or alternatively to find new typesof catalysts that are not susceptible to poisoning.

Another growing trend in emission control is to address the problem ofemissions from small engines, such as those fitted to motorcycles,construction equipment and garden machinery. These engines present atwo-fold problem for conventional PGM catalysts. Firstly, they tend tohave relatively high levels of engine-out emissions. Secondly, thelength of their exhaust systems is short. The combination of these twofactors means that the catalyst is exposed to very high temperatures(resulting from the exhaust temperature on entering the catalyst zoneand the heat generated by exothermic reactions on ihe catalyst). Thehigh temperatures induce a variety of undesirable effects inconventional PGM catalysts. The most common of these is sintering, andothers include loss of surface area and changed interactions between thePGMs and the catalyst. Carol et al (High Temperature Deactivation ofThree-Way Catalyst; Soc. of Automotive Engineers; paper 892040) foundthat ageing a Pt:Rh TWC for 48 hours at 1323° K reduced its conversionefficiency for hydrocarbons and CO by almost 50%.

Additionally there could at some time be concern about emissions of PGMsfrom catalysts.

The three most important advantages of the catalysts of the inventionover a conventional PGM TWC are:

-   -   increased resistance to poisoning;    -   increased tolerance of high-temperature operation;    -   absence of PGM emissions to the atmosphere.

The dispersed MM′O_(z), phase that is produced in these doped aluminacatalysts can involve Pb, Zn, K, Na, etc as integral components. Hencethese SPA materials are not poisoned by elements normally a problem forPGM-based TWCs.

Compared with simple perovskites (e.g. U.S. Pat. Nos. 5,939,354 and5,977,017 describe perovskite samples of modest surface area [<13 m²/g])the catalysts of the invention have a higher surface area. They areamorphous and readily wash-coated onto a variety of suitable supports.They contain no platinum group metals, unlike some perovskites which maybe selected to contain Ru, Co, Ni and Pd. The use of some of theseperovskites further causes specific concerns over nickel emissions,either as the metal or its compounds.

The invention further provides a method of preparing a three waycatalyst for the removal of pollutants from the exhaust gases ofautomotive internal combustion engines which comprises forming a sol-gelof alumina doped with cations of other metals, in which the dopantscomprise cations of two groups of metals M and M′ as defined above. Inthese sol-gels M and M′ are both present in a quantity in the range 10to 25% by weight of the catalyst and the weight ratio of M:M′ lies inthe range 0.5-2.0.

The sol-gels of the invention are typically opaque gels of variableviscosity. Sol-gel processing of the doped and undoped alumina isbeneficial in yielding high surface areas, for example 140-150 m²/gafter heating to 1273° K, and allowing uniform distribution of dopantssuch as BaO. A high available surface area is an especially desirablecharacteristic of automotive exhaust treatment catalysts. Having Ba²⁺ asM has the advantage of enhancing NO_(x), storage under oxygen-richconditions, in addition to decomposing-reducing NO_(x).

The sol-gels incorporating catalysts according to the invention may beapplied as a coating to a suitable substrate, for example bywash-coating, dip coating, spin coating or spray coating, either assingle or multiple layers. The preferred substrates are monolithicceramic or monolithic metallic materials. In one very useful embodimentof the invention the sol-gel is formed in situ on the substrate, therebyavoiding any need for a pre-coating step and generally facilitating—andthus reducing the cost of—the exhaust gas treatment system in which itis incorporated.

According to the method of the invention the sol-gel may conveniently beprepared by the following steps:

-   -   (a) a salt (e.g. a nitrate) of M and a salt (e.g. a nitrate) of        M′ in a selected ratio and selected concentrations are dissolved        in an organic complexing agent (e.g. a glycol of suitable        OH-group separation), and the resulting solution is refluxed;    -   (b) as aluminium alkoxide is dissolved in a further quantity of        the complexing agent used for M and M′ and this solution is also        refluxed;    -   (c) the refluxed solutions of (a) and (b) are mixed and further        refluxed;    -   (d) the mixed solution is diluted in a controlled manner by        addition of a specified quantity of water and the refluxing is        continued;    -   (e) the diluted mixed solution is aged in the presence or        absence of a pore templating agent and is then either        -   (i) diluted with alcohol (corresponding to the Al alkoxide            used) to a suitable viscosity to provide a sol-gel suitable            for coating on to a substrate; or        -   (ii) dried (in vacuum, sub—or super-critically) and calcined            to give a homogeneous material.

In a variation of the aforesaid sol-gel preparation procedure, sols ofMO_(x) and M′O_(x) sols may be introduced to the sol-gels at selectedtimes through the procedure to give a less-homogeneous,partially-segregated material.

In an alternative embodiment of the invention, the sol-gelpairwise-doped alumina catalysts may be prepared in-situ on thesubstrate by organo-metallic chemical vapour deposition.

The invention is further described with reference to the following tableand FIGS. 1 to 3.

FIG. 1 illustrates the activity of a commercial three way catalyst withand without 1% Pb—PbO_(x) addition.

FIG. 1(a) shows the results for CO conversion.

FIG. 1(b) shows the results for C₃H₈ conversion. In both cases theair:fuel ratio (λ) was 1. Blank data for homogeneous oxidation reactionsare given by open dotted circles; these reactions had very low rates.

FIG. 1 shows that a sample (200 mg) of a ground commercial Pd, Pt and RhPGM-containing three way catalyst (TWC) on cordierite was active in COand C₃H₈ oxidation when tested under stoichiometric conditions chosen tobe standard (i.e. 6000 ppm CO, 1000 ppm NO, 520 ppm propane, 5800 ppmO₂, N₂ balance to 101 kPa, flowing at 60,000 h⁻¹). The temperaturesrequired for 50% CO and C₃H₈ conversion [T_(1/2)(CO) and T_(1/2)(C₃H₈)]are 550° K and 855° K respectively. The addition of 1% Pb—PbO_(x)suppressed CO oxidation, but (surprisingly) not C₃H₈ oxidation activity.The T_(1/2)(CO) and T_(1/2)(C₃H₈) values for the homogeneous reaction inthe blank reactor were much higher (i.e. 940° K and 890° Krespectively).

The table illustrates results for homogeneous samples prepared with M=Ceand M′=Ba. This shows that the total surface areas (e.g. 111-162 m²/g)of the samples after thermal treatment at 1173-1223° K were good,although the total surface areas did decrease as the total dopantconcentration rose. TABLE % M = Ba % M′ = Ce S_(BET) (m²/g) 0 5 144 5 5145 20 5 118 0 15 162 5 15 150 20 15 111The M-M′ dopants used alone did not suppress the alumina surface areaunduly.

FIG. 2(a to d) illustrates oxidation of CO (a,b) and propane (c,d) oversol-gel exhaust catalysts with different Ce (M′) and Ba (M) levels.

Unsupported samples 200 mg) of sol-gel pairwise doped alumina (SPA)catalysts were tested for their potential as non-PGM TWC components inCO and propane (C₃H₈) oxidation (see FIG. 2) using the standardstoichiometric reactant stream mentioned above with reference to FIG. 1.

At 5% ceria, BaO addition produced poor CO oxidation activity relativeto the TWC (see FIG. 2 a), but at 15% ceria, BaO addition produced agood CO oxidation catalyst, with a lower T_(1/2)(CO) than the commercialTWC (see FIG. 2 b). The best CO oxidation catalysts had about 20% CeO₂and BaO, beyond this the surface area and activity of the SPA catalystwas suppressed. However, the best SPA sample had better low temperatureactivity than the TWC, although this moved to higher temperatures as thespace velocity increased.

In C₃H₈ oxidation the addition of baria was shown to be more importantthan ceria. Again the overall propane oxidation activity of the best SPAsample was better than the commercial TWC: its T_(1/2)(C₃H₈) being lowerthan that of the PGM-TWC [e.g. 850° K in FIG. (1 b)].

Raising the space velocity to 120,000 h⁻¹ over the non-PGM SPA materialsraised their T_(1/2) (CO) and T_(1/2)(C₃H₈) values as it would for aPGM-based TWC. However, the lower cost of non-PGM SPA materials meansthere is less of an economic and environmental penalty in raising thecatalyst loading or weight to maintain activity at higher spacevelocities. Similar trends to those seen in FIG. 2 were found for otherhydrocarbons and at other λ ratios. SPA non-PGM oxide materials have bydesign a variable oxygen content and z value. It is this OSC propertyand their oxygen buffening capacities (OBC) which allows a broadening ofthe λ—window over which they operate.

The table also shows that the SPA catalysts of the invention comparewell in terms of surface area with earlier doped aluminas (e.g. thosedescribed in the Angrove paper mentioned above) and perovskites (e.g.Golden's U.S. Pat. Nos. 5,939,354 and 5,977,017 mentioned above).

FIG. 3 shows the beneficial effects upon CO and C₃H₈ and propaneoxidation activity of barium and cerium addition to sol-gelpairwise-doped alumina samples. Such catalysts also showed usefulactivity in NO_(x) removal on Ba—Ce pair-wise doping.

1-10. (Cancelled).
 11. A catalyst for a removal of pollutants fromexhaust gases of an automotive internal combustion engine whichcomprises alumina doped with cations of other metals, wherein thedopants comprise cations of two groups of metals M and M′, where M andM′ are one of: (a) monovalent M and pentavalent M′, (b) divalent M andquadravalent M′, (c) divalent M and a combination of divalent andpentavalent M′, (d) divalent M and a combination of trivalent andpentavalent M′, (e) divalent M and a combination of divalent andhexavalent M′, (f) trivalent M and trivalent M′, and (g) trivalent M anda combination of divalent and quadravalent M′, and wherein each of M andM′ is present in a quantity in a range of 10 to 25% by weight of thecatalyst and the weight ratio of M:M′ lies in a range of 0.5 to 2.0. 12.A catalyst as claimed in claim 11, wherein in the form of a sol-gel ofalumina doped with the said cations of other metals.
 13. A catalyst asclaimed in claim 12, wherein the alumina doped with the cations of othermetals is in a homogenous phase in the sol-gel.
 14. A catalyst asclaimed in claim 11, wherein the weight ratio of M:M′ is substantially1:1.
 15. A catalyst as claimed in claim 12, wherein the weight ratio ofM:M′ is substantially 1:1.
 16. A catalyst as claimed in claim 13,wherein the weight ratio of M:M′ is substantially 1:1.
 17. A catalyst asclaimed in claim 11, wherein the catalyst is deposited on one of amonolithic ceramic and a metallic substrate.
 18. A catalyst as claimedin claim 12, wherein deposited on one of a monolithic ceramic andmetallic substrate.
 19. A catalyst as claimed in claim 13, wherein thecatalyst is deposited on one of a monolithic ceramic and a metallicsubstrate.
 20. A method of preparing a catalyst for a removal ofpollutants from the exhaust gases of automotive internal combustionengines, comprising the steps of: forming a sol-gel of alumina dopedwith cations of other metals, the dopants comprising of cations of twogroups of metals M and M′, where M and M′ are one of: (a) monovalent Mand pentavalent M′, (b) divalent M and quadravalent M′, (c) divalent Mand a combination of divalent and pentavalent M′, (d) divalent M and acombination of trivalent and pentavalent M′, (e) divalent M and acombination of divalent and hexavalent M′, (f) trivalent M and trivalentM′, and (g) trivalent M and a combination of divalent and quadravalentM′, and wherein each of M and M′ is present in a quantity in a range of10 to 25% by weight of the catalyst and the weight ratio of M:M′ lies ina range of 0.5 to 2.0.
 21. A method as claimed in claim 20, wherein thesol-gel is formed in situ on a support substrate.
 22. A method asclaimed in claim 20, wherein the sol-gel is prepared by the followingsteps: (a) a salt of M and a salt of M′ in a selected ratio and selectedconcentrations are dissolved in an organic complexing agent, and theresulting solution is refluxed; (b) aluminium alkoxide is dissolved in afurther quantity of the complexing agent used for M and M′ and thissolution is also refluxed; (c) the refluxed solutions of (a) and (b) aremixed and further refluxed; (d) the mixed solution is diluted in acontrolled manner by addition of a specified quantity of water and therefluxing is continued; and (e) the diluted mixed solution is aged inthe presence or absence of a pore templating agent and is then either(i) diluted with alcohol, corresponding to the Al alkoxide used, to asuitable viscosity to provide a sol-gel suitable for coating on to asubstrate; or (ii) dried in vacuum, sub-or super-critically and calcinedto give a homogeneous material.
 23. A method as claimed in claim 21,wherein the sol-gel is prepared by the following steps: (a) a salt of Mand a salt of M′ in a selected ratio and selected concentrations aredissolved in an organic complexing agent, and the resulting solution isrefluxed; (b) aluminium alkoxide is dissolved in a further quantity ofthe complexing agent used for M and M′ and this solution is alsorefluxed; (c) the refluxed solutions of (a) and (b) are mixed andfurther refluxed; (d) the mixed solution is diluted in a controlledmanner by addition of a specified quantity of water and the refluxing iscontinued; and (e) the diluted mixed solution is aged in the presence orabsence of a pore templating agent and is then either (i) diluted withalcohol, corresponding to the Al alkoxide used, to a suitable viscosityto provide a sol-gel suitable for coating on to a substrate; or (ii)dried in vacuum, sub-or super-critically and calcined to give ahomogeneous material.
 24. A method as claimed in claim 20, wherein solsof M Ox and M′ OX sols are introduced to the sol-gels at selected timesthrough the procedure to give a less-homogeneous, partially-segregatedmaterial.
 25. A method as claimed in claim 21, wherein sols of M Ox andM′ OX sols are introduced to the sol-gels at selected times through theprocedure to give a less-homogeneous, partially-segregated material. 26.A method as claimed in claim 22, wherein sols of M Ox and M′ OX sols areintroduced to the sol-gels at selected times through the procedure togive a less-homogeneous, partially-segregated material.
 27. A method asclaimed in claim 23, wherein sols of M Ox and M′ OX sols are introducedto the sol-gels at selected times through the procedure to give aless-homogeneous, partially-segregated material.
 28. A method as claimedin claim 24, wherein sols of M Ox and M′ OX sols are introduced to thesol-gels at selected times through the procedure to give aless-homogeneous, partially-segregated material.
 29. A method as claimedin claim 20, wherein the sol-gel is prepared in-situ on a substrate byorgano-metallic chemical vapour deposition.